1. Field of the Invention
The invention relates to the field of optical detection systems for target detection, optical communications, imaging, and range estimation. More particularly, this invention relates to the field of quantum illumination detection systems for stealthy target detection, stealthy optical communications, imaging, and range estimation.
2. Background Information
Optical or optoelectrical systems have long been used in various sensing applications, including but not limited to target detection, communications, imaging, and range estimation. Target detection systems include a transmitter for interrogating or illuminating a target region, and a receiver for detecting a return signal representative of the presence or absence of an object in the target region. Communications systems include a transmitter for transmitting a modulated signal to a receiver across a noisy channel. The receiver receives a noisy modulated signal and deciphers the modulated signal from the presence of noise.
Some target detection systems employ coherent-light laser beams in the transmitter, the receiver, or in both the transmitter and receiver. These systems are collectively referred to as Laser Radar or Laser Detection and Ranging (LADAR) systems. In the simplest LADAR target detection system, for example, the presence of a target is detected by the transmitter shining a laser beam towards the target region, and the receiver determining whether any of the transmitted laser light is reflected back. Laser beams are created by stimulating the emission of light “photons” from atoms. As these atoms lose energy, they emit photons, which are collected and transmitted as a laser beam. Coherent laser beams include photons which have a fixed phase relationship with one another. The phase relationship may be temporal, spatial, or spatio-temporal.
In optical communication systems that use laser beams, the laser beam to be transmitted by the transmitter is modulated and/or encoded so that this laser beam carries information that is transmitted to a receiver. The receiver then demodulates and/or decodes the received laser beam.
However, for both targets and for receivers located far away from the source of the transmitted laser beam, only a small fraction of the transmitted laser beam is reflected back from the target region to the receiver. In addition, if the target region contains other light sources or thermal radiation (collectively referred to as “noise” sources), it may be very difficult to distinguish the component of the return signal containing light reflected off the target from that contributed by noise, because the return signal-to-noise ratio of the return light beam is very low. While this noise decreases the probability of detection of a target by a target detection system, in optical communication systems, the presence of this noise may be exploited for stealthy communication of messages from the transmitter to the receiver.
A different kind of state of light, called quantum-mechanically entangled light, can be created by nonlinear crystals which are pumped by lasers. Quantum entanglement refers to the phenomenon that the quantum mechanical state of one photon in the pair is correlated with the quantum mechanical state of the other photon in the pair in a way that is stronger than any classical system. For instance, if the polarization state of one of the photons is known, then the polarization state of the other photon is known. Or perhaps, if the frequency or wavelength of one photon is known, then the frequency or wavelength of the other photon is known, too. Or perhaps, if the phase of one photon is known, then the phase of the other photon is known, too.
Recent research into a method called “quantum-illumination” predicts that with the use of quantum-mechanically entangled light to interrogate or illuminate distant objects, significant enhancements may be achieved over the use of unentangled/coherent light for detecting those objects. However, only one known detection system exists for realizing these theoretical predictions of enhancement. This system primarily uses quantum-mechanically phase-entangled light, is described in commonly-owned U.S. patent application Ser. No. 12/610,739 (Published U.S. Patent Application No. 20100177297), the contents of which are incorporated herein in their entirety. Therefore, there is a need for other practically realizable joint-detection optical receivers that exploit this significant quantum-mechanical enhancement in stealthy target detection, stealthy optical communications, imaging and range estimation systems.